instanceof和类型转换

1.instanceof：判断类型是否相似

2.System.out.println(X instanceof Y);//能不能编译通过！取决于X和Y直接是否存在父子关系，有父子关系编译通过，无父子关系编译报错。

　　X指向的类型是否是Y的子类型

eg：

//Object > String
//Object > Person > Teacher
//Object > Person > Student
Object object = new Student();
//System.out.println(X instanceof Y);//能不能编译通过！
System.out.println(object instanceof Student);//true
System.out.println(object instanceof Person);//true
System.out.println(object instanceof Object);//true
System.out.println(object instanceof Teacher);//false
System.out.println(object instanceof String);//false
System.out.println("==================================");
Person person = new Student();
System.out.println(person instanceof Student);//true
System.out.println(person instanceof Person);//true
System.out.println(person instanceof Object);//true
System.out.println(person instanceof Teacher);//false
//System.out.println(person instanceof String);//编译报错！
System.out.println("==================================");
Student student = new Student();
System.out.println(student instanceof Student);//true
System.out.println(student instanceof Person);//true
System.out.println(student instanceof Object);//true
//System.out.println(student instanceof Teacher);//false
//System.out.println(student instanceof String);//false
System.out.println("==================================");

3.子类转换为父类，可能丢失自己的本来的一些方法！

4.父类引用指向子类的对象

　　Person person = student;

5.把子类转换为父类，向上转型；

6.把父类转换为字类，向下转型；需要强制转换，可能会丢失方法

7.方便方法的调用，减少重复的代码！简洁